SAMMY Wilson told party colleague Edwin Poots that he was "astounded and appalled" by his attitude towards the Northern Ireland Protocol as a minister, in a private letter sent earlier this year.
Details of the stinging criticism, which occurred days before Mr Poots' short stint as DUP leader between May and June, emerged following a Freedom of Information request made by the BBC.
The letter, which was sent by East Antrim MP Mr Wilson and focussed on checks on pets travelling from Britain to the north, demanded to know why the agriculture minister had not "resisted the views of his officials" when it came to implementing the protocol.
The DUP is opposed to the protocol as it places a trade border in the Irish Sea in order to prevent a land border in Ireland.
A previous letter sent from Mr Poots, whose department is responsible for overseeing border control posts at ports to check goods, outlined to Mr Wilson the requirements for people taking pets across the Irish Sea.
Routine checks on the non-commercial movement of pets is delayed to at least October - a move made by Mr Poots following receipt of the critical letter - but new documentation will then be required as the EU Pet Travel Scheme no longer applies in Britain following its departure from the bloc.
In his response to the letter from his party colleague, Mr Wilson wrote that he was "astounded and appalled at the response I received from you."
The MP also expressed disbelief that Mr Poots had not pursued "remedies which are open to you".
He continued in his correspondence: "I am surprised that you are permitting discussions to go on within your department to look at how this part of the protocol could be implemented when your party's position is that there should be no implementation of the protocol and indeed it should be replaced.
"I trust that you will reconsider the answer which you have given to me and give some hope to those who are looking for political leadership to destroy this protocol rather than meekly bow to the unreasonable demands which it makes."
